A Direct Digital Frequency Synthesizer (DDFS) is a digitally-controlled signal generator that may vary the output signal frequency over a large range of frequencies, based on a single fixed-frequency precision reference clock. In addition, a DDFS is also phase-tunable. In essence, within the DDFS, discrete amplitude levels are fed to a Digital-to-Analog Converter (DAC) at a sampling rate determined by the fixed-frequency reference clock. The output of the DDFS provides a signal whose shape depends on the sequence of discrete amplitude levels that are fed to the DAC at the constant sampling rate. The DDFS is particularly well suited as a frequency generator that outputs a sine or other periodic waveforms over a large range of frequencies, from almost DC to approximately half the fixed-frequency reference clock frequency.
A DDFS offers a larger range of operating frequencies and requires no feedback loop, thereby providing near instantaneous phase- and frequency changes, avoiding over- and undershooting and settling time issues associated with another analog systems. A DDFS may provide precise digitally-controlled frequency and/or phase changes without signal discontinuities.
Polar Modulation is related to inphase (I) and quadrature (Q) modulation in the same way that polar coordinates are related to the Cartesian coordinate system. For polar modulation, the orthogonal I and Q components of an RF signal are converted to a phasor representation comprising an amplitude component and a phase component. In this way, the combined I and Q signal may be generated with one phase change and one amplitude change, whereas separate I and Q modulation may require amplitude and phase modulation for each channel, especially for non-constant envelope modulation modes. In addition, the I and Q modulation approach may require good linearity of the power amplifier, often leading to power inefficient designs that suffer from parameter variability due to factors such as temperature. In contrast, polar modulation may allow the use of very efficient and non-linear amplifier designs for non-constant envelope modulation schemes.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.